In general, unshielded twisted pair cables each comprising a predetermined number of insulated conductor-pairs (defined "pairs" hereinafter) stranded by a predetermined lay-length and a protective sheath covering the stranded pairs are used in a LAN (Local Area Network) system. The unshielded twisted pair cables are installed in a building vertically floor to floor, or horizontally in spaces of ceilings, that is, plenums without using metal conduits.
In such installation state, there is a possibility in which the unshielded twisted pair cables carry fire in case where a fire spread out in a building. Therefore, the unshielded twisted pair cable is required to have flame-retarding properties.
A conventional unshielded twisted pair cable comprises a predetermined number of pairs insulated with polyethylene or fluorine resin, and a protective sheath of polyvinyl chloride (PVC) covering the pairs, wherein the fluorine resin insulations and the PVC sheath provide flame-retarding properties.
In the conventional unshielded twisted pair cable, however, there are disadvantages in that smoke and harmful gases including halogen gases such as hydrogen chloride, hydrogen fluoride, etc. are generated from the insulated pairs and the protective sheath at the time of fire, so that human bodies are badly affected, evacuation and extinguishing activities are obstructed due to the hindered views, and a computer network, a communication equipment, etc. are deteriorated by corrosive gases. There is a further disadvantage in the conventional unshielded twisted pair cable in that a high frequency leakage current flows through the sheath to increase a transmission loss at a high frequency band, because the pairs are not shielded under the situation where the transmission of high speed digital signals ranging 10 Mb/s to 100 Mb/s (TPDDI LAN) is required in accordance with the requirement of high speed LAN systems in recent years. Furthermore, conventional unshielded twisted pair cables have the fluctuation of transmission characteristics, and the difference of laying condition. This problem is caused that these cables are laid in a buildings under various conditions as described above.